@mixin col-6() {
    width: 50%;
    float: left;
}
@mixin col($width: 50%) {
    @if type-of($width) != number {
        @error "$width必须是一个数值型，你输入的是：#{$width}.";
    }
    @if not unitless($width) {
        @if unit($width) != "%"{
            @error "$width应该是个百分值，你输入的width是：#{$width}.";
        }
    } @else {
        @warn "$width应该是个百分值，你输入的width是：#{$width}.";
        $width: (percentage($width) / 100);
    }
    @media (min-width:768px){
        width: $width;
        float: left;
    }
}